Control valves (e.g., sliding stem valves, rotary valves, etc.) are commonly used in process control systems to control the flow of process fluids. Sliding stem valves such as, for example, gate valves, globe valves, etc., typically have a valve stem (e.g., a sliding stem) that moves a flow control member (e.g., a valve plug) disposed in a fluid path between an open position to allow fluid flow through the valve and a closed position to prevent fluid flow through the valve. A control valve typically includes an actuator (e.g., a pneumatic actuator, hydraulic actuator, etc.) to automate the control valve. In operation, a control unit (e.g., a positioner) supplies a control fluid (e.g., air) to the actuator to position the flow control member to a desired position to regulate the flow of fluid through the valve. The actuator may move the flow control member through a complete stroke between a fully closed position to prevent fluid flow through the valve and a fully open position to allow fluid flow through the valve.
In practice, many control valves are implemented with fail-safe or override systems. A fail-safe override system typically provides protection to a process control system by causing the actuator and, thus, the flow control member to move to either a fully closed or a fully open position during emergency situations, power failures, and/or if the control fluid (e.g., air) supply to an actuator (e.g., a pneumatic actuator) is shut down.
At the closed position, the flow control member engages a valve seat disposed within the valve to prevent fluid flow through the valve. In the closed position, the actuator provides a force to impart a seat load to the flow control member to maintain the flow control member in sealing engagement with the valve seat. In high pressure applications (e.g., high pressure process fluid at an inlet of the valve), the seat load provided by the actuator may be insufficient to maintain the flow control member in sealing engagement with the valve seat, thereby resulting in undesired leakage through the valve. Providing an adequate or sufficient seat load or opening force is particularly important when the valve is in a failed position. In a failed position, the actuator causes the flow control member to move to a predetermined position (e.g., the fully closed position, the fully open position).
Air-based (e.g., pneumatic) fail-safe systems are often implemented with double-acting control actuators to provide a fail-safe or override mechanism. In operation, air-based (e.g., pneumatic) fail-safe systems may be configured to compensate for the lack of sufficient force (e.g., seat load or opening force) provided by an actuator. However, such known air-based fail-safe systems require additional components (e.g., volume tanks, trip valves/switching valves, volume boosters, etc.), thereby significantly increasing complexity and costs.
Other known actuators (e.g., spring-return actuators) provide a mechanical fail-safe mechanism. These known actuators may use an internal spring in direct contact with a piston to provide a mechanical fail-safe to bias the piston to one end of the stroke travel (e.g., fully opened or fully closed) when the control fluid supply to the actuator fails. However, when used with long-stroke applications (e.g., stroke lengths of four (4) inches or more), such long-stroke spring-return actuators often provide poor control. That is, in some applications, the spring rate of the bias or fail-safe spring may be sufficient to degrade actuator performance because the supply fluid and the control member must overcome the bias force of the fail-safe spring. In practice, long-stroke actuators often use a return spring having a smaller or lower spring rate to accommodate the long-stroke length (i.e., so that the spring can compress the length of the stroke). However, in these long-stroke actuators, the lower spring rate often results in insufficient seat load or force to cause the flow control member to sealingly engage a valve seat to prevent leakage through the valve (or to fully open to allow fluid flow through the valve) upon a system failure, thereby providing an inadequate fail-safe system.